K-shell ionization in the electron and positron decays of 64Cu

The total K-ionization probabilities accompanying the β+ and β- decays of 64Cu, P/sub K/(β+), and P/sub K/(β-) were determined from coincidence and singles K x-ray spectra of Ni and Zn to be (13.2 +- 0.8) x 10-4 and (11.9 +- 0.8) x 10-4, respectively, using a high resolution Si(Li) photon spectrometer. P/sub K/(β+)/P/sub K/(β-) was measured directly and independently to be 1.26 +- 0.10, in fair agreement with the ratio of the separate measurements 1.11 +- 0.10. While the average ratio is in agreement with the shaking theory of Isozumi, Shimizu, and Mukoyama, the individual values are a factor of approx. 1.5 larger than their predictions. A similar approximately twofold excess over the Isozumi, Shimizu, and Mukoyama hydrogenic-wave-function-based predictions has been observed for almost all cases of K-shell shaking in β- decay, and now also for this first reported measurement in β+ decay. The measured ratio is the first experimental evidence affirming the previously recognized and acknowledged error of a factor of 2 (excess) for shakeoff in β- decay (but not in β+ decay) in the theory of Law and Campbell. No evidence was found for a postulated Ni K vacancy production mechanism, in which the positron annihilates with a K electron internal to the parent atom, exceeding a few percent of K shaking in positron decay